Today polyester films are widely used as the base material for magnetic recording tapes because of their excellent mechanical and thermal properties as well as their dimensional stability.
In the field of magnetic recording tape, there is a demand of video recording tapes is rapidly increasing. Although the characteristics required of the video recording tape vary greatly, high density recording performance is strongly desired for prolonged time recording in tapes used with home video sets.
In order to carry out high density recording, employment of shorter recording wavelength and a narrower track is required, which inevitably tends to reduce play-back output and increase noise to signal ratio.
In this case, it is required to make close the contact between the video head and the tape in order to decrease the so-called spacing loss. In this connection, a base film as smooth as possible is required in order to make the magnetic surface as smooth as possible.
However, as the film surface is made smoother, slidability which is another important property required of the base film, tends to be impaired. Especially in the case of prolonged time recording, where the running speed of the tape is made very slow, the tape must run at a stable speed. Moreover, stability in the running speed when a tape is repeatedly used, is required.
Polyester films are usually manufactured by melt-extruding a polyester material in a sheet and biaxially stretching the sheet longitudinally and transversely, and finally winding it into a roll. In this case, however, troubles often occur in winding-up the film if the film surface is smooth. For instance, end edges of a roll become irregular, or creases, wrinkles or flaws occur in the wound-in part of the sheet or the wound-up surface.
Under such circumstances, we searched for a process for manufacturing a polyester film, which would be excellent in windability (wound-up characteristics) during film making, and would provide the magnetic tape produced therefrom with satisfactory electromagnetic properties, and that would have excellent slidability in repeated use. As a result, we have found that a polyester film satisfying the above property requirement to a high degree can be obtained by incorporating a specific kind of particles in a film and furnishing the film with a surface roughness which satisfies specific conditions. This invention was accomplished on the basis of this discovery.